A modern, integrated perspective on the evolution of bee morphology

Bees comprise a diverse group of insects, which have been evolving in association with angiosperms for the last ~125 My. This striking association, along with the bees' diversity of morphological features and life histories, has made them one of the best-studied groups of insects to date. There are seven main lineages of bees, traditionally recognized as families. Among these seven families, five are referred to as 'short-tongued' bees and two as 'long-tongued' bees. These families have considerable disparities in morphological diversity and species richness, but there is limited knowledge of what factors influenced the emergence of these disparities. Here, we study bee morphological disparity, phylomorphospace occupancy, and diversification using discrete morphological data in light of a robust phylogeny inferred through UCE phylogenomics. Our results suggest that the emergence of the long-tongued bees' mouthparts opened a new area of morphospace and, putatively, a new adaptive zone for the group. This arises mainly because this clade was able to expand its occupancy of the morphospace considerably, with no detectable shift in diversification rates. Conversely, early diversification of Halictidae, one of the five short-tongued families and the second most species-rich family, is interpreted as a case of nonadaptive radiation because, despite an increase in the diversification rates along its stem branch, the group has low morphological disparity and occupies a small area of the morphospace. These results provide new perspectives on early bee diversification dynamics and morphological evolution and reveal that different evolutionary phenomena shaped the diversity of bees.